The sinuses are a connected system of hollow cavities in the skull. The largest sinus cavities are about an inch across. 

– Your cheekbones hold your maxillary sinuses (the largest). – The low-center of your forehead is where your frontal sinuses are located.

– Between your eyes are your ethmoid sinuses. – In bones behind your nose are your sphenoid sinuses.

They’re lined with soft, pink tissue called mucosa. Normally, the sinuses are empty except for a thin layer of mucus.

The inside of the nose has ridges called turbinates. Normally these structures help humidify and filter air.

A thin wall, called the septum, divides the nose. Most of the sinuses drain into the nose through a small channel or drainage pathway that doctors call the “middle meatus.”

Why do we have sinuses? Experts don’t know. One theory is that they help humidify the air we breathe in. Another is that they enhance our voices.

Acute sinusitis (sinus infection): Viruses, bacteria, or fungi infect the sinus cavity, causing inflammation. More mucus; nasal congestion; discomfort in the cheeks, forehead, or around the eyes; and headaches are common symptoms.